fix digit escape in highlight regex for constants

[the-mikedavis]

Checklist: (:heavy_check_mark:)

• All tests pass in CI (they pass locally but I can't get the Actions CI running right now 🤔)
• There are sufficient tests for the new fix/feature (added one)
• Grammar rules have not been renamed unless absolutely necessary (N/A)
• The conflicts section hasn't grown too much (N/A)
• The parser size hasn't grown too much (N/A)

This is the same idea as tree-sitter/tree-sitter-java#90, the \d should be \\d to become the digit escape.

This one is a bit more straightforward: the query currently misconstrues constants with digits in them as constructors (because of the rule below it).

pi.php

<?php
define("PI", 3);
define("PI_314", 3.14);
echo PI;
echo PI_314;
?>

query1.scm

((name) @constant
 (#match? @constant "^_?[A-Z][A-Z\d_]+$"))

query2.scm

((name) @constant
 (#match? @constant "^_?[A-Z][A-Z\\d_]+$"))

reproduction

$ tree-sitter query query1.scm pi.php
pi.php
  pattern: 0
    capture: constant, start: (3, 5), text: "PI"

$ tree-sitter query query2.scm pi.php
pi.php
  pattern: 0
    capture: constant, start: (3, 5), text: "PI"
  pattern: 0
    capture: constant, start: (4, 5), text: "PI_314"

[maxbrunsfeld]

Thank you!